1. Field of the Invention
The present invention relates to techniques for reducing, in a radio communications system, interference from another radio communications system.
2. Description of Relevant Art
There is known an IMT-2000 (International Mobile Telecommunications-2000) system enabling high-rate communications of 2 Mbps at maximum, in the field of mobile radio communications, such as portable telephone communications, where spreading needs are observed for high-rate data communications.
The IMT-2000 system provides two standards of communications system, an IMT-2000/FDD (Frequency Division Duplex) system and an IMT-2000/TDD (Time Division Duplex) system.
The IMT-2000/FDD system employs, as access methods therefore, a CDMA (Code Division Multiple Access) and an FDMA (Frequency Division Multiple Access). The system is advantageous, to the IMT-2000/TDD system, in that it can have a wider service area, and is expected to be used principally for low-rate data communications about 384 kbps as well as audio communications.
The IMT-2000/TDD system employs, as access methods therefore, the CDMA and the FDMA, and besides a TDMA (Time Division Multiple Access). This system is advantageous in that the ratio between up-going traffic and down-going traffic can be freely changed, and is expected to be used principally for high-rate data communications above 384 kbps.
FIG. 1 schematically diagrams a radio communications system, which has an IMT-2000 system and a PHS (Personal Handyphone System) system mixed in an identical service area, so that an IMT-2000/FDD system and an IMT-2000/TDD system mixed in an identical service area.
As shown in FIG. 1, the IMT-2000 system is configured with an IMT-2000 radio terminal 10, a TDMA radio base station (IMT-2000/TDD radio base station) 20, a non-TDMA radio base station (IMT-2000/FDD radio base station) 30 and a radio network controller 40.
The IMT-2000 radio terminal 10 is provided with a TDMA radio (IMT-2000/TDD radio) 11, a non-TDMA radio (IMT-2000/FDD radio) 12, and a controller 13.
The TDMA radio 11 performs radio communications with a TDMA radio base station 20 via radio channels of the IMT-2000/TDD system. The non-TDMA radio 12 performs radio communications with a non-TDMA radio base station 30 via radio channels of the IMT-2000/FDD system. The controller 13 is connected to, for controlling, the TDMA radio 11 and the non-TDMA radio 12.
The IMT-2000 radio terminal 10, provided with both a function of the IMT-2000/TDD system (TDMA radio 11) and a function of the IMT-2000/FDD system (non-TDMA radio 12), has both an advantage of the IMT-2000/TDD system, which is adapted for high-rate data communications, and an advantage of the IMT-2000/FDD system, which is adapted for wide service area.
The TDMA radio base station 20 performs radio communications with the TDMA radio 11 of the IMT-2000 radio terminal 10 via radio channels of the IMT-2000/TDD system. The TDMA radio base station 20 is connected to a radio network controller 40. Further, the TDMA radio base station 20 is adapted, for indication of a possible call connection via radio channels of the TDMA system, to always transmit a control signal indicating this effect.
The non-TDMA radio base station 30 performs radio communications with the non-TDMA radio 12 of the IMT-2000 radio terminal 10 via radio channels of the IMT-2000/FDD system. Also the non-TDMA radio base station 30 is connected to the radio network controller 40. Further, the non-TDMA radio base station 30 is adapted, for indication of a possible call connection via radio channels of the non-TDMA system (FDMA system or CDMA system), to always transmit a control signal indicating this effect.
The control signal transmitted from the TDMA radio base station 20 and the control signal transmitted from the non-TDMA radio base station 30 are different from each other.
The radio network controller 40, connected to the TDMA radio base station 20 and the non-TDMA radio base station 30, is adapted for controlling them to allot radio channels to the IMT-2000 radio terminal 10. The radio network controller 40 is further connected, via an IMT-2000 network 1, to other radio network controllers, as well as to a PHS network 2, a stationary telephone network, a mobile telephone network, etc.
As shown in FIG. 1, the PHS system is configured with a PHS radio base station 60 and a PHS radio 70. The PHS radio base station 60 and the PHS radio 70 are connected to each other via radio channels. Further, the PHS radio base station 60 is connected, via a PHS network 2, to other PHS radio base stations, as well as to the IMT-2000 network 1, a stationary telephone network, a mobile telephone network, etc.
FIG. 2A shows the configuration of radio channels employed in the IMT-2000 system. As shown in FIG. 2A, the radio channels are configured with radio frames 1301 to 130n, the radio frames being respectively configured with radio slots 1311 to 13115.
Each radio frame 1301 to 130n has 15 radio slots 1311 to 13115, as in FIG. 2A. The radio frames 1301 to 130n, have a time length of 10 ms.
In the IMT-2000/FDD system, every radio slot 1311 to 13115 of a respective radio frame 1301 to 130n has a single radio channel allotted thereto. In the IMT-2000/TDD system, every radio slot 1311 to 13115 of a respective radio frame 1301 to 130n can have a different radio channel allotted thereto.
FIG. 2B shows the configuration of radio channels employed in the PHS system. As shown in FIG. 2B, the radio channels are configured with radio frames 1321 to 132n, the radio frames being respectively configured with radio slots 1331 to 1338.
Each radio frame 1321 to 132n has 8 radio slots 1331 to 1338, as in FIG. 2B. The radio frames 1321 to 132n have a time length of 5 ms.
The IMT-2000 system and the PHS system, if mixed in an identical service area as described, have mutual interference therebetween. In particular, the mutual interference increases, in case the non-TDMA radio base station 30 and the PHS radio base station 60 are located near to each other, or when the non-TDMA radio 12 of the IMT-2000 radio terminal 10 and the PHS radio terminal 70 perform communications in vicinities thereof.
Description is made below of the mutual interference between the IMT-2000/FDD system and the PHS system.
The IMT-2000/FDD system is a radio communications system of a non-TDMA system, which always has radio waves transmitted from a non-TDMA radio 12 of an IMT-2000 radio terminal 10 and a non-TDMA radio base station 30.
The PHS system is a radio communications system of a TDMA system, which has radio waves transmitted, like a burst with a period of 5 ms, from a PHS radio terminal 70 and a PHS radio base station 60.
As a result, the IMT-2000/FDD system undergoes, with the period of 5 ms, burst-like interference from PHS system, with a deteriorated quality of radio communications.
As a solution, a known method increases signal transmission power of the non-TDMA radio 12 of the IMT-2000 radio terminal 10 or the non-TDMA radio base station 30, when the interference from the PHS system is increased at the IMT-2000/FDD system.
However, this method, in which the burst-like interference from the PHS system is short in time, tends to suffer from the influence of a delay between from when interference from the PHS system is measured at the IMT-2000/FDD system to when signal transmission power is increased at the non-TDMA radio 12 of the IMT-2000 radio terminal 10 or the non-TDMA radio base station 30, so that the method has a small reduction in deterioration of radio communications quality, as a problem.
Further, the PHS system suffers from increased interference from the IMT-2000/FDD system, as the signal transmission power is increased at the non-TDMA radio 12 of the IMT-2000 radio terminal 10 or the non-TDMA radio base station 30, as another problem.